In the eight major taboos of employing troops, the greatest harm comes from having no weak points. The saying, 'besiege with a weak point', means never to provoke the enemy into a do-or-die determination.
Therefore, since ancient times, when sieging a city, it was advised to surround on three sides and leave one open. Under such circumstances, it's possible to make the enemy commander waver between breaking through or fighting to the death, while also causing the soldiers' morale to scatter. After all, if there's a way out, who would choose to fight to the death?
More importantly, leaving a weak point is not the same as neglecting it, but instead, it involves setting up an ambush at the critical points through which the enemy must escape, causing them to fall into an ambush during their hasty breakout.
